Data from Earth-viewing satellite systems have advanced to the point that marine scientists can carry out global analyses that were inconceivable just a few years ago. An excellent example can be found in the papers by Uz et al. on page 597 of this issue1 and by Cipollini et al.2 in the 15 January issue of Geophysical Research Letters. Both groups find a significant covariation of global chlorophyll concentrations with satellite determinations of sea level. The upshot of their investigations is the recognition that so-called planetary or Rossby waves provide a physical mechanism for bringing nutrients to the surface layers of the open ocean and feeding phytoplankton productivity.
Like all plants, phytoplankton require both light and nutrients to grow. But only in the upper 50 to 150 m of the water column of the open sea is there enough light for phytoplankton photosynthesis to occur. In this thin, well-lit euphotic zone, available nutrients are rapidly assimilated. Typically, then, it is the supply of new nutrients from deeper water that limits productivity in the open ocean.
In most coastal and high-latitude regions of the sea, vertical mixing and upwelling provide sufficient vertical fluxes of nutrients to sustain phytoplankton growth. In contrast, the surface layers of much of the subtropical and tropical ocean are oligotrophic (nutrient-poor). In such regions, intense upper-layer stratification through a feature known as the thermocline seals off the euphotic zone from below, inhibiting the upward transport of nutrients. For these regions, diffusion of nutrients by small-scale turbulence is thought to be inadequate to account for observed rates of productivity3, 4. So some other physical process must be stirring up the oligotrophic ocean and supplying nutrients to the euphotic zone.
A leading candidate for such a disturbance is 'eddy pumping'4, 5. In the most general terms, ocean eddies and meanders on scales of tens to hundreds of kilometres drive the vertical motion of water, and so have important biological and biogeochemical consequences, including lifting subsurface nutrients into the euphotic zone4, 5, 6, 7, 8. In the stratified mid-latitude ocean, the formation of cyclonic mesoscale eddies, which are closed vortices roughly 200 km in diameter, are thought to be the dominant physical process driving the eddy-pumping flux4, 5, 9.
So, is that the end of the eddy pumping story? This is where the work of Uz et al.1 and Cipollini et al.2 comes in. They find that there is a global statistical coherence between sea level, detected by satellite, and ocean chlorophyll concentration on scales of 300–1,000 km — considerably larger than are typical for mesoscale eddies. Chlorophyll provides a proxy measurement for phytoplankton biomass and hence productivity; sea-level variations primarily reflect changes in the depth of the thermocline. By analysing the spatial scales and propagation velocities of the eddies, the authors interpret these variations as baroclinic planetary waves or Rossby waves. Baroclinic Rossby waves are large-scale (a few hundred to several thousand kilometres across), westward-propagating waves, which create vertical displacements of the themocline of 10–100 m, and which are driven by the latitudinal change in the Coriolis force10. They have long been known to be 'messengers' of oceanic climate disturbances. But the ability of satellites to measure sea level to within a few centimetres has added fresh impetus to research into the characteristics and dynamics of these waves11, 12.
The demonstration by Uz et al. and Cipollini et al. of a link between Rossby waves and global chlorophyll distribution has a bearing on the eddy-pumping hypothesis. For example, mesoscale eddies should retain water masses within their core. So nutrients should reach the euphotic zone as the eddy forms, not as it propagates. On the other hand, planetary waves are waves in the classical sense and do not transport water masses with them. So they should be able to lift subsurface nutrients continuously into the euphotic zone as they propagate across an ocean basin9. In a sense, they may act like a rototiller, or rotovator, 'turning over' upper-ocean nutrients in their path. This fundamental difference between waves and eddies will have an important bearing on how physical observations are used to infer eddy-pumping fluxes9.
Uz et al.1 find that only 10 to 20% of the variance in chlorophyll (filtered to remove seasonal and other variations) is explained by the sea-level signal. So it seems that the 'Rossby rototiller' may not be the dominant physical process regulating eddy pumping. Further research is also needed to understand how variability in chlorophyll biomass translates to the cycling of carbon and associated materials in the water column. There is a long way to go before a predictive understanding of the interactions of planetary waves and mesoscale eddies on phytoplankton productivity and, more generally, ocean biogeochemical cycles, is established13.
